As a method for producing hydrogen through thermochemical water-splitting using a heat of not lower than 1,000° C. that is obtained by concentrating solar light, a two-step thermochemical water-splitting cycle using a metal oxide such as an iron oxide and a cerium oxide has been expected to become a promising one. Accordingly, institutes in various countries have worked on developing reactors for the two-step thermochemical water-splitting.
Inventors of the present invention once developed a solar reactor internally circulating iron-oxide particles and conducted an experiment with a prototype of the solar reactor (See, non-patent documents 1 to 5). The solar reactor, as shown in FIG. 8, includes a quartz window provided on the ceiling of the reactor and introduces solar light from the quartz window. Such solar light is downwardly concentrated by a beam-down typed light collecting system as a solar concentrating system. In the reactor, the solar light is irradiated onto the internally circulating fluidized bed made of metal oxide particles, thereby heating the particles. Further, the inventors also developed metal oxides used in the two-step thermochemical water-splitting (See, patent document 1).
The two-step thermochemical water-splitting using the metal oxides is performed by alternately repeating two reactions: one reaction is an oxygen evolution reaction which releases oxygen from the metal oxides in a high temperature of not lower than 1,400° C. under a low-oxygen partial pressure gas atmosphere such as nitrogen atmosphere; and the other reaction is a hydrogen evolution reaction which generates hydrogen by allowing the metal oxides with oxygen having been released therefrom to come into contact with water vapor in a low temperature of not higher than 1,400° C. The following reaction formulas express reactions of the oxygen and hydrogen evolution that occur when using NiFe2O4 as the metal oxide.NiFe2O4→3Ni1/3Fe2/3O+½O2 3Ni1/3Fe2/3O+H2O→NiFe2O4+H2 
According to the conventional method using the solar reactor developed by the inventors of the present invention, a low-oxygen partial pressure gas such as nitrogen is at first flowed to a metal oxide particles bed within the solar reactor from the bottom of the metal oxide particles bed, thereby preparing an internally circulating fluidized bed. Next, the internally circulating fluidized bed is irradiated with concentrated solar light in order to heat the particles to 1,400° C. or higher, thus causing the oxygen evolution reaction. After that, the temperature of the particles is decreased to 1,400° C. or lower by reducing the aunt of the concentrated solar light introduced into the solar reactor, and the flowed gas from the bottom of the fluidized bed is switched to water vapor, thereby causing the hydrogen evolution reaction.